1. Field of the Invention
The present invention relates to a reflection type photodetection apparatus suitable for detecting the intensity of the reflection of an emitted light reflected by a detected object without being affected by outside light, and relates further to a biological information measuring apparatus comprising this reflection type photodetection apparatus for measuring a pulse wave, pulse, the pitch of body movement or other biological information.
2. Description of Related Art
Devices for measuring biological information such as the pulse and body motion include electronic devices for optically detecting a change in blood volume to display biological information based on the detected result. This type of optical pulse wave measuring device (biological information measuring apparatus) emits light from an LED (light emitting diode) or other light emitting element to the finger tip, for example, and detects light reflected from the body (blood vessel) by means of a photodiode or other light detecting element. It is therefore possible to detect a change in blood flow produced by the blood pulse wave as a change in the amount of detected light. The change in the pulse rate or pulse wave is then displayed based on the pulse wave signal thus obtained. Infrared light is conventionally used as the light emitted from the light emitting element.
It should be noted here that when outside light such as natural light or fluorescent light is incident on the photodetector, the amount of detected light fluctuates with the variation in the incidence of outside light. More specifically, the fingertip or other detected part is typically covered by a light shield in a conventional biological information measuring apparatus to suppress the effects of outside light because this outside light is noise (external disturbance) to the pulse wave signal to be detected.
The luminance of natural light is, however, significantly greater than the luminance of light emitted from the light emitting element when directly exposed to natural light, such as when outdoors. A problem with a conventional biological information measuring apparatus is, therefore, that when it is used where exposed to outside light, such as outdoors, some of the outside light inevitably passes though the finger tissues and reaches the photodetector no matter how large the light shield for blocking outside light is made, and pulse detection errors resulting from variations in the luminance of outside light occur easily. Such conventional biological information measuring apparatuses are therefore limited to use in places where they are not exposed to outside light, or where the luminance of any outside light is constant. This limitation can be overcome by using an even larger light shield structure, but the size of the biological information measuring apparatus then cannot be reduced.
To resolve this problem, Japan Unexamined Utility Model Application Publication (jikkai) S57-74009 (1982-74009) teaches a pulse wave sensor comprising, in addition to a pulse wave detector for detecting a pulse wave, an outside light detector for detecting outside light. This outside light detector is covered with a filter having the same transmission characteristics as the body tissues so that the pulse wave sensor can compensate for the effects of outside light based on the result of outside light detection by the outside light detector.
There are, however, individual differences in the transmission of outside light, and it is therefore difficult using the above-noted technology to accurately compensate for an outside light component. Furthermore, the path of outside light to the pulse wave detector varies according to the relative positions of the pulse wave detector and the finger. That is, each time the detection device is used, the path length from the point of incidence of outside light on the tissue to the pulse wave detector changes. It is therefore not possible to accurately compensate for an outside light component even by providing a filter with constant transmission characteristics.
A conventional device for detecting the pitch of body movement typically uses a built-in acceleration detector to detect movement of the body, and determines the pitch of body movement from the body movement signal. A pedometer, for example, uses a piezoelectric element PZT as a compact acceleration detector, and detects the speed at which the user is moving by applying wave shaping to the detected body movement signal.
Devices combining the above-noted acceleration detector and an optical pulse wave sensor are also available as portable pulsimeters capable of measuring the pulse while the user is exercising. Such portable pulsimeters apply a fast Fourier transform process (FFT) to the body movement signal detected by the acceleration detector and the pulse wave signal detected by the optical pulse wave sensor to separately detect a body movement spectrum indicative of the body movement signal and a pulse wave spectrum indicative of the pulse wave signal. The pulse wave spectrum and body movement spectrum are then compared, the frequency component corresponding to the body movement spectrum is removed from the pulse wave spectrum, and the frequency with the greatest spectrum power is then removed from the remaining spectrum to determine the fundamental frequency of the pulse wave signal. The pulse rate is then calculated based on the fundamental frequency of the pulse wave signal. A conventional pulsimeter therefore applies two FFT operations, and calculates the pulse rate based on the results of these FFT operations.
The present inventors have also proposed in Japanese Patent Application H5-241731 (1993-241731) a device enabling pulse rate detection while the user is exercising using only an optical pulse wave sensor and not using an acceleration detector. This device focuses on the difference in the absorption characteristics of oxygenated hemoglobin in arterial blood and reduced hemoglobin in venous blood. The operating principle of this device uses the long wavelength (e.g., 940 nm) of the absorption characteristic of oxygenated hemoglobin compared with the absorption characteristic of reduced hemoglobin, and the long wavelength (e.g., 660 nm) of the absorption characteristic of reduced hemoglobin compared with the absorption characteristic of oxygenated hemoglobin to detect pulse wave signals, applies a FFT operation to both pulse wave signals, and determines the fundamental frequency of the pulse wave signals by comparing the results of the FFT operations.
Small, low cost acceleration detectors used in pedometers are sensitive in only one direction, therefore cannot detect movement in all directions, and thus cannot accurately detect body movement. This problem can be resolved using a acceleration detector with three axes, but this results in a more complex construction, and makes it difficult to reduce the size.
A further problem with the above-described pulsimeters that use an acceleration detector is that it is not possible to continue detecting the pulse rate while exercising if the acceleration detector fails. In addition, whether or not an acceleration detector is used, conventional pulsimeters require two FFT operations, thereby resulting in a more complex configuration and requiring a further process to determine the fundamental frequency of the pulse wave signal from the frequency analysis result.